Transactional vs. marketing emails differ in trigger and scope, with transactional emails triggered by individual customer actions and marketing emails sent to lists of contacts without a specific event. Transactional emails typically have higher engagement rates, faster delivery speeds, and are less susceptible to human error, while marketing emails often require recipient permission and can be more prone to errors. Separating these email types is crucial for better deliverability, reduced support requests, and improved brand trust. This separation can be achieved by using different IP addresses, subdomains, or even just sending from different email addresses, and tools like Message Streams can help streamline the process. By separating marketing and transactional emails, businesses can optimize each type of email without interference and improve their overall email strategy.